For industrial pressure-sensitive adhesive tape applications it is very common to use polyacrylate pressure-sensitive adhesives. Polyacrylates possess a variety of advantages over other elastomers. They are very stable toward UV light, oxygen, and ozone. Synthetic and natural rubber adhesives generally contain double bonds, which render these adhesives unstable to the aforementioned environmental effects. A further advantage of polyacrylates is their transparency and their usefulness across a relatively wide temperature range.
Polyacrylate pressure-sensitive adhesives are generally prepared in solution by means of a free-radical polymerization. The polyacrylates are, generally speaking, coated from solution onto the corresponding carrier material, via a coating bar, and subsequently dried. To increase the cohesion the polymer is crosslinked. Curing proceeds thermally or by UV crosslinking or by EB curing (EB: electron beams). The operation described is relatively cost-intensive and environmentally objectionable, since as a general rule the solvent is not recycled, and a high level of consumption of organic solvents implies a high environmental burden.
Furthermore, it is very difficult to produce pressure-sensitive adhesive tapes at high application rate without bubbles.
Remediation of these drawbacks is implied by the hotmelt process. Here, the pressure-sensitive adhesive (PSA) is applied from the melt to the carrier material.
This new technology, however, also entails restrictions. Prior to coating, the solvent is removed from the PSA, which additionally is prepared in solution, in a drying extruder. The drying operation is associated with a relatively high temperature and shearing exposure, so that high molecular weight polyacrylate PSAs in particular are significantly damaged. The acrylate PSA undergoes gelling or the low molecular weight fraction is sharply enriched as a result of molecular weight reduction. Both effects are unwanted, since they are deleterious for the application. Either the adhesive can no longer be coated, or its technical adhesive properties are altered.
One solution for lessening these drawbacks is offered by polyacrylate adhesives having a low average molecular weight and a narrow molecular weight distribution. Here, the fraction of low molecular weight and high molecular weight molecules in the polymer is sharply reduced as a result of the polymerization process. The disappearance of the high molecular weight fractions lowers the flow viscosity, and the composition shows less of a tendency to gel. As a result of the lowering of the low molecular weight fraction, there is a reduction in the number of oligomers, which reduce the shear strength of the PSA.
For producing low molecular weight PSAs a variety of polymerization methods are suitable. State of the art is the use of regulators, such as, for example, alcohols or thiols (Makromoleküle, Hans-Georg Elias, 5th edition, 1990, Hüthig & Wepf Verlag Basle). These regulators reduce the molecular weight but broaden the molecular weight distribution.
A further controlled polymerization method used is that of atom transfer radical polymerization, ATRP. The various possibilities of ATRP are described in U.S. Pat. No. 5,945,491 A, U.S. Pat. No. 5,854,364 A and U.S. Pat. No. 5,789,487 A. Generally speaking, metal catalysts are used, a side-effect of which is a negative influence on the aging of the PSAs (gelling, transesterification). Moreover, the majority of metal catalysts are toxic, discolor the adhesive, and can be removed from the polymer only by means of costly and inconvenient precipitations.
U.S. Pat. No. 4,581,429 A discloses a controlled free-radical polymerization process. The process employs as its initiator a compound of the formula R′R″N—O—X, in which X represents a free radical species which is able to polymerize unsaturated monomers. The conversion rates of the reactions, however, are generally low. A particular problem is the polymerization of acrylates, which proceeds only at very low yields and molecular weights.
WO 96/24620 A, WO 98/30601 A, and WO 98/4408 A describe further polymerization methods in which regulating substances are used to prepare polymers having low polydispersities. Disadvantages of these methods include the low conversion and the use of solvents.
In the aforementioned patents or papers an attempt is made to improve the control of free-radical polymerization reactions. There exists, nevertheless, a need for a nitroxide-controlled polymerization method which is highly reactive and with which high conversions are achievable in conjunction with high molecular weight and low polydispersity.
Another version is the RAFT process (Reversible Addition-Fragmentation Chain Transfer). The process is described at length in WO 98/01478 A and WO 99/31144 A, but in the manner depicted therein is not suitable for the production of PSAs, since the conversions achieved are very low and the average molecular weight of the polymers produced is too low for acrylate PSAs. Hence the polymers described cannot be used as acrylate PSAs. An improvement is achieved with the process described in DE 100 30 217 A.
Neither the method according to the RAFT process nor the use of nitroxide compounds, however, can be employed for UV crosslinking, since the compounds disclosed possess a radical scavenger effect, so that the crosslinking efficiency following addition of the free UV photoinitiator is too low.
Guse (U.S. Pat. No. 4,144,157 A) describes a process in which the acrylate PSAs are readily UV-crosslinkable and can be processed as a hotmelt, and yet, owing to the broad molecular weight distribution, they do not possess good technical adhesive properties. A further disadvantage is that these PSAs are also produced by solution polymerization with subsequent removal of solvent.
Commercially, low molecular weight acrylate hotmelts are offered in which benzophenone derivatives or acetophenone derivatives have been incorporated as an acrylated photoinitiator into the acrylate polymer chain. They can then be crosslinked with UV-radiation (see also U.S. Pat. No. 5,073,611 A). The shear strength achievable with such systems, however, is not satisfactory, since for a relatively low molecular weight (Mw (weight average) about 250 000 g/mol) these systems have a relatively broad distribution. A great disadvantage of these systems, moreover, is that they still contain measurable fractions of residual solvent and hence are not solvent-free.
An improvement to acrylate PSAs in connection with their processing by the hotmelt method and subsequent UV crosslinking is achieved through the production of narrow-distribution acrylate PSAs with copolymerized photoinitiators—as set out in DE 101 49 084 A. The use of copolymerized photoinitiators significantly increases the crosslinking efficiency as compared with the use of promoters which promote UV crosslinking and are not added until after the polymerization. With this method, however, the polymerization still takes place in solution and the polymer melt is produced only by concentration down to a residual solvent content of <2%. The environmental and also economic problem described at the outset, namely the high level of consumption of organic solvents, is therefore not solved here either. A further factor is that possible solvent residues in the adhesive can lead to odor nuisance in the course of subsequent use.
A solventless method for producing polyacrylates in a twin-screw extruder is described in EP 0 160 394 A. However, the acrylate hotmelt PSAs produced by that method have a high gel fraction, in some cases considerably so, of up to 55%, thereby severely impairing their further processing. Since, here as well, UV crosslinking is only possible through subsequent addition of promoters, the crosslinking efficiency, too, is low.
The central problem which therefore remains is the production of solvent-free acrylate PSAs with an efficient UV crosslinking.
It is an object of the invention, therefore, to provide a method for producing a solvent-free polyacrylate composition which can be processed very effectively by the hotmelt method and thereafter is very effectively crosslinkable, and also to provide the resultant acrylate hotmelt PSAs, which does not have the drawbacks of the cited prior art, or has them only to a reduced extent.